Question

An optically pure compound X gave an ${\left[\alpha \right]}_D^{25}=+{20.0}^0.$ A mixture of (X) and its enantiomer (Y) gave ${\left[\alpha \right]}_D^{25}=+{10.0}^0.$ Select the true ($T$)/ false ($F$) statements regarding this mixture.

(a) It has 50% dextrorotatory isomer.

(b) It has 25% laevorotatory isomer.

(c) It has 50% optical purity.

• A. $TTF$
• B. $FFT$
• C. $FTT$
• D. $TFT$

Answer 1

Answer: (C)

$FTT$

% Optical purity of mixture $=\frac{{\left[\alpha \right]}_{obs}}{{\left[\alpha \right]}_{pure}}\times 100$
$=\frac{10}{20}\times 100=50$%
Hence statement (c) is true.

$d-l=50$

and $d+l=100$

$2d=150$

$\therefore d=7$5%

Hence statement (a) is false.

$75+l=100$

$l=25$%

Hence statement (b) is true.

Therefore option C (FTT) is correct.